It has been known that particulate matters (PM) contained in exhaust gas discharged from an internal combustion engine (e.g., a diesel engine) are collected by a dedicated filter (i.e., Diesel Particulate Filter (DPF)). Since the regulation for the exhaust gas has become stricter recently, further purification of the exhaust gas is desired. Accordingly, an exhaust gas aftertreatment device desirably includes a reduction-causing catalyst that purifies nitrogen oxides in the exhaust gas in addition to the DPF.
A reduction-causing agent in a form of ammonia obtained from a urea aqueous solution is used for such a reduction-causing catalyst Ammonia in a state of a urea aqueous solution is pumped from a tank to an injector and injected from the injector into an exhaust pile (a mixing pipe) on an upstream of the reduction-causing catalyst. The injected urea aqueous solution is thermally decomposed by the heat of the exhaust gas and ammonia obtained by the thermal decomposition is supplied to the reduction-causing catalyst.
The urea aqueous solution accumulated in the tank is pumped to the injector through a urea aqueous solution pipe. Since such a urea aqueous solution pipe is installed in an engine compartment, the urea aqueous solution may be deteriorated by thermal influence from the engine. Particularly, in some working vehicles, an inside of an exterior cover is divided into an engine compartment and a heat exchanger compartment in which a radiator and the like are arranged. In such a case, a cooling air supplied from a cooling fan to the radiator and the like does not flow into the engine compartment. Consequently, a temperature of the engine compartment is further significantly increased, whereby acceleration of deterioration is concerned.
For this reason, it has been typically proposed to provide a cooler in a supply pipe or a return pipe that returns the urea aqueous solution from the injector to the tank (see, for instance, FIGS. 1 and 2 of Patent Literature 1).